Cellulose can be saccharified by various methods. Among them, enzymatic saccharification, which has low energy consumption and a high sugar yield, has been chiefly developed. Cellulase, a cellulolytic enzyme, is roughly classified into cellobiohydrolase, which acts on the crystalline region of cellulose, and endoglucanase, which acts internally on cellulose molecular chain to reduce the molecular weight. These cellulases are known to be inhibited by one of their products, cellobiose. Beta-glucosidase is an enzyme that acts on water-soluble oligosaccharide or cellobiose and catalyzes hydrolysis of the β-glycosidic bond. In particular, β-glucosidases are essential enzymes to sufficiently obtain glucose, which is a useful fermentation raw material. It is known that the reaction of cellobiohydrolase and endoglucanase is inhibited by accumulation of cellobiose generated by cellulose decomposition. That is, the β-glucosidase can significantly reduce accumulation of cellobiose generated by cellulose decomposition and thereby has an effect of notably improving cellulose decomposition efficiency.
Cellulose is contained in herbaceous plants and arboreous plants in large amounts. These plants are collectively called “cellulose-containing biomass.” The cellulose-containing biomass contains hemicellulose such as xylan and arabinan, and lignin, in addition to cellulose. In particular, the lignin contained in cellulose-containing biomass is an aromatic polymer compound and is known to have inhibitory activity on enzymatic saccharification using filamentous fungal-derived cellulase. Though the inhibitory mechanism of lignin on the filamentous fungal-derived cellulase has not completely been elucidated, adsorption of cellulase to lignin is believed as one of factors of reducing the decomposition efficiency (P. Hetti et al., Journal of Biotechnology, 107, 65-72 (2004)).
Thermostable enzymes have high stability and retain activity for a long time even under high temperature conditions, and the use thereof as industrial enzymes has been investigated. Such thermostable enzymes have been confirmed to be highly present as enzymes possessed by thermophilic bacteria or hyperthermophilic bacteria.
Thermostable β-glucosidases also have been identified from several thermophilic bacteria or hyperthermophilic bacteria. Specifically, thermostable β-glucosidases have been identified from microorganisms such as Pyrococcus furiosus, Pyrococcus horikoshii, Thermotoga maritima, Sulfolobus shibatae, Sulfolobus solfataricus, and Clostridium thermocellum. In particular, it is disclosed that the β-glucosidase derived from Clostridium thermocellum forms a monomer (P. Christian et al., Trichoderma and Gliocladium: Basic Biology, Taxonomy and Genetics., Vol. 1, 121-138 (1998)) and that the β-glucosidase derived from Sulfolobus solfataricus or Pyrococcus furiosus forms a tetramer (H. Ohba et al., Biosci. Biotech. Biochem., 59, 1581-1583 (1995) and MW Bauer et al., J. Biol. Chem., Vol. 271, 39, 23749-23755 (1996)). The relationship between these structures and their functions has not yet been revealed.
It could therefore be helpful to provide a β-glucosidase having high enzyme activity of hydrolyzing cellulose-containing biomass.